One of the consequences of the generalization of Internet connections at private users' premises is the appearance of consumer voice telephony services that make use of this connection. In the last few years, videophone products and services have appeared as increased connection speeds allow good quality video telephony. The most advanced videophones are cordless phones, generally according to the wireless standard IEEE802.11, also referred to as WiFi.
Several protocols have been developed with the purpose to manage the establishment and monitoring of telephony connections, of which the protocol Session Initiation Protocol (SIP) has become the dominant protocol, both for voice and video telephony. SIP is a signaling protocol for initiating, managing and terminating voice and video sessions across packet networks.
A conventional communication using the SIP protocol is depicted in FIG. 1. It comprises a first wireless videophone A and a second wireless videophone B.
For a wireless videophone to connect to the network NET, an intermediary device must be used which is connected on one side to the wireless videophone via a wireless connection (e.g. WiFi connection) and on the other side to the network via a suitable IP (stands for Internet Protocol) connection (e.g. ADSL, cable). This intermediary device is often referred to as a wireless gateway, wireless modem, wireless modem/router, and will be referred to as a wireless base station BS_A or BS_B in the following.
For one videophone A to call another videophone B, specific SIP management devices need to be present in the network that provides the connection between these two phones. A basic implementation of such a network consists of a single SIP management device, referred to as a SIP server SERV in the following.
The main task of the SIP server is to detect requests for calls from the wireless videophones, and provide these videophones with the required information that allows a connection between two phones to be established. One of the most important information required for establishing a SIP connection consists of the IP addresses and the SIP identifiers of both videophones. The calling videophone knows the SIP identifier (e.g. a telephone number) of the called videophone, but not its IP address. The SIP server delivers this IP address.